1. Field of the Invention
This invention relates to a liquid crystal display device wherein a liquid crystal layer is held between a pair of electrode substrates and more particularly to a liquid crystal display device whose a liquid crystal layer is divided into a plurality of domains between which liquid crystal molecules are set in different tilt directions.
2. Description of the Related Art
Liquid crystal display devices are applied in various fields of OA equipments, information terminals, watches, televisions and the like because of their characteristics of lightness, thinness and low power consumption. Particularly, an active matrix type liquid crystal display panel has an excellent response characteristic obtained by thin film transistors (TFTs) for switching pixels. Thus, the active matrix type liquid crystal display panel is used as a monitor display for a portable TV or a computer, which is required to quickly display a large quantity of image information.
In recent years, with an increase in quantity of information, there is a demand for enhancement of the resolution and display speed of the liquid crystal display panel. As a technique of enhancing the resolution and display speed of the liquid crystal display panel, the number of increased by miniaturization of the TFT array structure. In this case, a liquid crystal display mode, which can provide a response speed of liquid crystal molecules twice to several tens of times higher than current level, is required to change the alignment of liquid crystal molecules in a period shortened according to the increase in the number of pixels. Such an liquid crystal display mode can be selected from Optically Compensated Birefringence (OCB), Vertically Aligned Nematic (VAN), Hybrid Aligned Nematic (HAN), and n-alignment modes using a nematic liquid crystal and Surface-Stabilized Ferroelectric Liquid Crystal (SSFLC) and Anti-Ferroelectric Liquid Crystal (AFLC) modes using a smectic liquid crystal, for example.
Particular attention has recently been paid to the VAN mode, which has a higher response speed than the conventional twisted nematic (TN) mode and adopts vertical alignment treatment, thus dispensing with a conventional rubbing process that may cause defects such as electrostatic destruction. Moreover, a design for viewing angle compensation becomes easier when the VAN mode is employed. The viewing angle can be enlarged by a multi-domain structure in which a liquid crystal layer of each pixel is divided into a plurality of domains between which liquid crystal molecules are set in different tilt directions.
When the multi-domain structure of the VAN mode is applied to pixels of the liquid crystal display device, the plurality of domains are created, for example, by distorting an electric field applied from each electrode to the liquid crystal layer, with a use of a slit formed inside the electrode or an insulating body formed periphery of the electrode, so that the tilt directions can be determined according to the dielectric constant anisotropy of a liquid crystal material and the distorted electric field.
FIG. 13 shows an example of the conventional liquid crystal display device in which a slit is formed to obtain a pixel with the multi-domain structure. A pixel electrode includes a plurality of electrode segments 1 which are separated by the slit 2 and set to the same potential by use of a bridge-wiring member 3. The bridge-wiring member 3 is integrally formed with the electrode segments 1 as a wiring which electrically connects the electrode segments 1 to each other on the same plane. Since the bridge-wiring member 3 is formed across a part of the slit 2, it causes an undesired distortion of the electric field defined in the liquid crystal layer by the slit 2. As a result, not only useful domains between which the tilt directions of liquid crystal molecules 4 depend on the slit 2 but also useless domains between which the tilt directions of the liquid crystal molecules 4 depend on the bridge-wiring member 3 may be caused by application of the electric field. When this is practically observed by use of a polarization microscope, a black line 5 which is called a schlieren texture is observed on the boundary between the above domains.
FIG. 14 shows the cross section taken along the XIV-XIV line of FIG. 13, for illustrating the alignment state of the liquid crystal molecules 4 adjacent to both ends of the bridge-wiring member 3. In this example, since electric flux lines 6 are not inclined as distortion of the electric field by the slit 2, the liquid crystal molecules 4 are aligned with those on the slit 2 side in two directions. FIG. 15 shows the cross section taken along the XV-XV line of FIG. 13, for illustrating the alignment state of the liquid crystal molecules 4 adjacent to both sides of the bridge-wiring member 3. In this example, the liquid crystal molecules 4 are influenced by the inclination of the electric flux lines 6, and are aligned in two directions that differ from those of FIG. 14. That is, the alignment of the liquid crystal molecules 4 is set in four directions within a local area including the bridge-wiring member 3.
When a mechanical shock is given to the liquid crystal display device by depressing the display screen with a finger during an image display operation, an irregular alignment of the liquid crystal molecules 4 occurs as shown in FIG. 16. Since the electric flux lines 6 are not inclined in positions adjacent to the both ends of the bridge-wiring member 3 in the cross section taken along the XVII-XVII line of FIG. 16, the liquid crystal molecules 4 can be aligned in any direction as shown in FIG. 17 from the alignment state is shown in FIG. 14 by external force. If the irregular alignment of the liquid crystal molecules 4 occurs in practice as shown in FIG. 16, this causes a phenomenon that the domain depending on the bridge-wiring member 3 becomes larger, and at the same time, the domain depending on the slit 2 becomes smaller. As a result, the black line 5 of the schlieren texture becomes longer as shown in FIG. 16 and gives an influence to the transmittance and the influence is visually observed as display irregularity on the display screen. If the electric field is kept applied for image display, it takes a long time for the liquid crystal molecules to return to the original alignment state in which no irregularity occurs, with a movement of misalignment called disclination. As a result, the display irregularity will be present until the liquid crystal molecules 4 return to the original alignment state shown in FIG. 14.
Further, in the liquid crystal display device of the CPA (Continuous Pinwheel Alignment) mode, each pixel is divided into a plurality of sub-pixels and the alignment of the liquid crystal molecules are set in rotation symmetry about the midpoint of the sub-pixel. The electric field is applied to the liquid crystal layer from a plurality of sub-pixel electrode segments of, for example, a square form having high symmetry. Thus, the same problem occurs when a bridge-wiring member is provided as wirings that electrically connect the sub-pixel electrode segments to set them to the same potential without requiring a redundant wiring space.